Star polymers and co-polymers have been prepared using degradable monomers such as D,L-lactide, glycolide, ε-caprolactone, δ-valerolactone, dioxanone, dioxepanone, trimethylene carbonate, and cyclic amides such as O-benzyl-L-serine (Schindler et al., Journal of Polymer Science; Polymer Chemistry Edition 20:319–326, 1982;. Storey et al., Polymer 38(26):6295–6301, 1997; Storey et al., Polymer 35(4).830–838, 1994; Bruin et al., Makromol. Chem. 9:589–594, 1998; Joziasse et al., Polymer 39(2):467–473, 1998; Li et al., Polymer 39(18):4421–4427, 1998; Kim et al., Makromol. Chem. 194:3229–3236, 1993; Kim et al., Makromol. Chem. 193:1623–1631, 1992; Hiljanen-Vainio et al., Journal of Biomedical Materials Research 34(1):39–46, 1997). These prepolymers have been thermally crosslinked to form elastomers using diisocyanate linkages (Storey et al., Polymer 35(4):830–838, 1994; Bruin et al., Makromol. Chem. 9:589–594, 1998) and methacrylate groups on the terminal ends (Storey et al., Polymer 38(26):6295–6301, 1997). However, diisocyanate crosslinked elastomers, depending on the diisocyanate used, have several disadvantages where biocompatibility/biodegradability are concerned. For example, they may degrade to potentially toxic compounds, they can only be crosslinked in solution, and they require a potentially carcinogenic solvent in order to achieve a dispersion of the crosslinking agent in the polymer. Also, use of such a solvent requires a further solvent removal step, and any residual solvent may jeopardize the biocompatibility of the material. Methacrylate end-capped star co-polymers have been cured to form elastomers, however, the reaction requires cobalt napthenate as a catalyst in an organic solvent. The catalyst raises concerns about biocompatibility as does the use of a solvent.
U.S. Pat. No. 3,072,680, issued Jan. 8, 1963, describes the synthesis of a number of bis-lactones. These compounds have been used to prepare elastomers by co-polymerization with monomers such as caprolactone and other lactones (U.S. Pat. No. 4,379,138, issued Apr. 5, 1983) and dioxepanone (Palmgren et al., J. Polym. Sci. A: Polym Chem. 35:1635–1649, 1997). This type of co-polymerization produces a random co-polymer whose crosslinks are strictly tetrafunctional. As this procedure provides little control over the structure of the prepolymers, there is high batch-to-batch variation in the characteristics of the resulting elastomers, making the physical properties and degradation kinetics of the elastomers difficult to reproduce.
Photo-crosslinking has been used to prepare elastomeric materials from acrylate tipped star polyurethanes (U.S. Pat. No. 5,674,921, issued Oct. 7, 1997). However, these urethanes are composed of monomers which produce toxic degradation products. Lactone star co-polymers composed of ε-caprolactone and trimethylene carbonate end tipped with coumarin (Matsuda et al., Macromolecules 33:795–800, 2000) have been crosslinked using long wave UV light, however these materials are rigid and brittle.